Studies were conducted on the effect of maternal immunization on the subsequent immune response of young mice to pneumococcal type 9V polysaccharide (PS), pneumolysin (ply), and 9V PS-ply conjugate. Young mice from mothers immunized with type 9V PS, inactivated pneumolysin, or 9V PS-ply conjugate during pregnancy received an additional dose of the same antigen at weaning. These mice produced higher specific IgM antibody than young mice from unimmunized mothers. Furthermore, young mice from mothers immunized with type 19F PS-ply conjugate during pregnancy that received an additional dose of the same conjugate after birth produced higher 19F PS IgG and IgM antibodies than young mice from unimmunized mothers. Rapid bacterial clearance following challenge with 5 x 104 cfu/dose of 19F pneumococci was observed in young mice that had received 19F PS conjugate during perinatal period as compared to the non-immunized controls. These results indicate that maternal immunization with pneumococcal PS-protein conjugate during pregnancy and administration of an additional conjugate to young mice after birth conferred protective immunity against pneumococcal infection in mice.